The role of metabolic activation in the toxicity and carcinogenicity of phenacetin is being studied. A carcinogenicity study has been initiated using femal Sprague-Dawley rats as the test animal. In this study, the effects of long term feeding of phenacetin are being tested. In addition, saccharin will be fed to test its promoting activity with respect to phenacetin. No data are yet available from this study. In vitro studies have utilized partially purified rabbit liver N,H-acytransferase as an activation system. N-Hydroxy-phenacetin is activated to a mutagen in the Ames test by this enzyme system, using Salmonella typhimurium strain TA-100, N-Hydroxy-phenacetin is an acetyl donor in the aminoazobenzene assay when incubated with rabbit liver acyltransferase. N-Hydroxy-phenacetin, radioactively-labeled at various positions, is bieng synthesized in order to further examine its enzymatic activation by acyltransfer and other pathways and the binding of active metabolites to macromolecules.